中医说的“望、闻、问、切”中的“切”，指的是号脉，这里的“脉”是指（　　）

两电阻串联在电路中，已知R₁:R₂=1:4，通过电阻的电流I₁:I₂之比是____ ；电阻两端的电压U₁:U₂之比是____ ；两端的电压与总电压U₁:U总之比是____ 。

   I am an English girl. My name is Mary. I’m twelve. My dad and mom are English teachers. I have a brother（哥哥）. His name is Tom and he is thirteen years old. We are in China now. We are in Beijing No. 4 Junior High School. Tom is in Class 4, Grade 7 and I am in Class 9, Grade 7. I have a dog. My dog looks strong. It has a big mouth and two big black ears. We have many(许多) Chinese friends here. They are good.

原价1680元的相机现在售价756元，相当于打（   ）出售。